1. Field of the Invention
The present invention relates to an optical fiber having the low-bending loss equal to that of a conventional trench structure at low cost.
2. Description of the Related Art
FTTH (Fiber To The Home) is in widespread use, accordingly, optical fibers have been installed indoors such as in buildings, households, and the like, and an optical fiber having reduced bending loss has been attracting attention.
By use of a bending loss insensitive fiber, the effect of preventing signals from being instantaneously interrupted due to loss generated by bending optical fibers, or the effect of reducing the laying cost due to easy handling of the optical fibers is expected.
There is a trade-off relationship between a mode field diameter (MFD) and bending loss in an optical fiber having a simple core-cladding structure which is used for a conventional single-mode optical fiber (S-SMF), it is possible to reduce the bending loss by reducing the MFD.
However, there is a problem in that the reduction of the MFD causes an increase in connection loss of S-SMF thereto or deviates from the range of the MFD determined by the international recommendation ITU-T G.652 related to a single-mode optical fiber (SMF) (the design criterion value at a wavelength of 1310 nm is a MFD of 8.6 to 9.5 μm), and there is a limitation in reducing the bending loss due to the reduction of the MFD.
As a technique of reducing the bending loss without reducing the MFD, a refractive index profile which is referred to as trench type is known (refer to Japanese Unexamined Patent Application, First Publication No. S63-43107).
Additionally, as a bending loss insensitive fiber, a number of methods using the trench-type refractive index profile has been proposed.
For example, PCT International Publication No. WO 2004/092794 discloses that the trench-type refractive index profile is not applied to a dispersion shifted optical fiber (DSF) disclosed in Japanese Unexamined Patent Application, First Publication No. S63-43107 but applied to a conventional SMF.
PCT International Publication No. WO 2006/025231 discloses a structure reducing the MFD and reducing the bending loss in addition to making the wavelength dispersion characteristics thereof to be the region similar to a normal SMF.
Published Japanese Translation No. 2010-503018 of PCT International Publication discloses a structure improving the reduction of the bending loss by making the relative refractive index difference of the trench region to be an extremely low value such as −0.63% or less.
Published Japanese Translation No. 2010-503019 of PCT International Publication discloses an optical fiber having a fiber cut-off wavelength less than 1260 nm and a zero-dispersion wavelength which is in the range of 1300 to 1324 nm and having reduced bending loss in the diameter of 10 mm.
As for shortening a manufacturing time of a trench optical fiber and designing for reducing the cost, a method of reducing a distance between a core and a trench is disclosed in Japanese Unexamined Patent Application, First Publication No. 2009-8850.
Japanese Unexamined Patent Application, First Publication No. 2007-279739 discloses a method of realizing both bending loss and single-mode transmission by providing an intermediate cladding coat and a low-refractive index layer outside of a trench coating.
Furthermore, a method is known which obtains the same effect as that of the trench-type by use of a constitution having air space at a part of a cladding coat (for example, refer to PCT International Publication No. WO 2004/092793 and Published Japanese Translation No. 2009-543126 of PCT International Publication).
In addition, as alternative approaches, a method is disclosed which releases the limitation of a cut-off wavelength by making the loss of a higher order mode increase, and obtaining a bending loss insensitive fiber (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2008-310328).
Moreover, Louis-Anne de Montmorillon, et al, “Recent Developments of Bend-insensitive and Ultra-bend-insensitive Fibers Fully Compliant with Both G.657.B and G.652.D ITU-T Recommendations”, Proceedings of the 58th IWCS/IICIT, International Wire & Cable Symposium, 2009, pp. 270-276 discloses an example related to behavior of a higher order mode in a trench optical fiber.
However, connection is an issue in a structure having a hole such as a hole-assisted fiber (HAF), Clear Curve (registered trademark), or the like, as compared with an optical fiber having a solid glass structure which does not have a hole such as a trench optical fiber.
For example, since accurate core alignment is required for a fusion splice, many fusion splicers use a direct core observation method which detects and aligns a core by analyzing an observed image of a side face of an optical fiber.
However, in a case where a hole is present in a cladding, since it is impossible to detect the position of the core by use of the image of the side face, an outer diameter alignment method with reference to the cladding diameter should be used.
Since the outer diameter alignment method is influenced by eccentricity of the core with respect to the circumscribed circle of the cladding, there is a problem in that connection loss easily increases as compared with a direct core observation method.
Additionally, the structure of the optical fiber disclosed in Japanese Unexamined Patent Application, First Publication No. 2008-310328 is extremely complicated, and an advanced manufacturing technique is necessary; furthermore, since a layer having a low refractive index is provided at the region which is wider than that of a trench structure, a large amount in dopants making a refractive index low is necessary, and there is a problem in terms of the cost of manufacturing.
Even in a case of the conventional trench structure, when the relative refractive index difference of the trench is set low such as to be approximately −0.7% or −0.5%, as exemplified in Published Japanese Translation No. 2010-503018 of PCT International Publication and Published Japanese Translation No. 2010-503019 of PCT International Publication, a large amount in dopants for forming a trench is necessary, and there is problem of an increase in the cost of manufacturing.
Although the optical fiber having a trench structure can be manufactured by a variety of manufacturing methods, suppression of the material cost for forming a trench coating is an issue depending on manufacturing methods.
FIG. 3 shows an example of the relationship between a partial pressure of silicon tetrafluoride (SiF4) in the case of forming a trench coating by use of a MVCD method, and the relative refractive index difference of the resultant fluorine-doped silica glass.
The relative refractive index difference of the resultant glass shows that it is proportional to approximately fourth root of a partial pressure of SiF4.
Consequently, in a structure which requires relative refractive index difference Δ such as less than −0.5%, the used amount of a source material gas of SiF4 dramatically increases.
For example, in order to obtain the relative refractive index difference Δ of 0.5%, it is necessary to set approximately 20 times partial pressure of SiF4 which is in the case of obtaining the relative refractive index difference Δ of −0.2%.
In Japanese Unexamined Patent Application, First Publication No. 2009-8850, a low-refractive index layer is designed so as to be close to a core in order to reduce the cost of materials; however, since a trench having a great negative relative refractive index difference should be provided in order to reduce bending loss, the effect for reducing the cost is limited.
Moreover, when the trench and the core are extremely close to each other, since optical characteristics such as chromatic dispersion or the like are separated from the international recommendation, there is a limitation to make the low-refractive index layer approach the core.
In contrast, there is a problem in that the loss of an optical fiber having a trench structure under a higher order mode is smaller than that of a conventional SMF.
The tendency shown as described above appears as difference in length dependence of a cut-off wavelength.
For example, Louis-Anne de Montmorillon, et al, “Recent Developments of Bend-insensitive and Ultra-bend-insensitive Fibers Fully Compliant with Both G.657.B and G.652.D ITU-T Recommendations”, Proceedings of the 58th IWCS/IICIT, International Wire & Cable Symposium, 2009, pp. 270-276 discloses the behavior of a trench optical fiber under a higher order mode, wavelength-dependence in LP11 Leakage Loss is shown in FIG. 2 thereof.
When wavelengths are compared to each other on the line of 1 dB/m corresponding to the cable cut-off wavelength λc22m of 22 m, the wavelengths of three optical fibers are distributed in the range of 1225 to 1260 nm, the optical fibers satisfying the cable cut-off wavelength less than 1260 nm defined by ITU-T or the like.
When losses are compared to each other in a wavelength of 1310 nm used for communication, the losses in optical fibers are approximately 2 to 12 dB/m.
Because of this, in the case of using an optical fiber having a short length such as several meters, the higher order mode does not sufficiently attenuate, and there is a possibility that communication is interrupted.
The foregoing behaviors are easily compared to each other when the difference between the cable cut-off wavelength λc22m of 22 m and the fiber cut-off wavelength λc2m of 2 m is represented as an indicator.
In the case of three optical fibers disclosed in Louis-Anne de Montmorillon, et al, “Recent Developments of Bend-insensitive and Ultra-bend-insensitive Fibers Fully Compliant with Both G.657.B and G.652.D ITU-T Recommendations”, Proceedings of the 58th IWCS/IICIT, International Wire & Cable Symposium, 2009, pp. 270-276, the difference is 63 to 146 nm.
In the case of designing a trench optical fiber so as to improve bending loss when a diameter is small such as a bending radius of 5 mm, the difference λc2m−λc22m has a tendency to be long.
For this reason, a desired structure has the equivalent bending property, and the difference in the cut-off thereof becomes low.